From EP 0 327 910 is known an injection device in which a dose is set in the classic way by rotating a tubular injection button engaging a threaded piston rod, thereby causing the injection button to elevate from the end of the injection device. By pressing down the injection button until abutment with a fixed stop, the threaded piston rod is moved a distance corresponding to the movement of the injection button. The piston rod mates a piston in a cartridge and medicine is expelled from the cartridge. This kind of injection device transmits the injection force directly to the piston of the cartridge but provides no gearing, i.e. the linear movement of the injection button corresponds exactly to the linear movement of the piston rod.
As a little amount of a medicament by directly transmitted injection force, causes a very little movement of the injection button, it is an advantage with a gearing ratio between the injection button and the piston rod to make it more clear to the patient, whether the injection is foregoing or not. Users having reduced finger strength could at the same time want a reduced injection force needed to carry out an injection.
EP 1 003 581 describes a number of methods to achieve a dose setting providing a gearing between the axial movement of the piston rod and the dose setting member. In one embodiment a dose setting member is rotated in a thread in the housing having a higher pitch than the pitch on the piston rod. When pressure is added to an injection button, the piston rod is being rotationally coupled to the dose setting member and as the piston rod is rotated in a nut fixed to the housing it is moved forward until the dose setting member abuts a fixed stop. This embodiment provides a gearing in movement, but does not reduce the needed injection force very much, as the transmission from linear movement to rotational movement and back from rotational to linear movement eats up most of the obtained force reduction due to friction.
A dose setting device, wherein a gearing provides both a longer injection button movement when setting the dose and a following lower injection pressure is described in WO 94/13344. This dose setting mechanism comprises a flat rotatable dose setting element engaged with an injection button, a dose administration wheel connected with a piston rod and a coupling ring connected with the dose setting element and injection button. Only when pressing the injection button, the coupling ring will engage the dose administration wheel, and as the gearwheel connection between the dose setting element and the injection button has a larger diameter than the gearwheel connection between the administration wheel and the piston rod, a gearing with a little loss of force due to friction is obtained. However, this development provides a complicated voluminous mechanical solution which demands high precision.
EP 1 568 389 describes a pen with a gearing with a very little loss of injection force due to friction. A nut is rotated along a threaded piston rod. The nut is provided with a gearwheel on the side, which on one side engages the housing and on the other side elevates an injection button. When the injection button is pushed, the nut and the piston rod are moved with a gearing ratio of 2:1 between the injection button and the piston rod. As none of the parts of the injection mechanism rotate in threads during injection, the loss of force due to friction is very limited. However, this mechanism is rather complicated, and it is not suited for simple, disposable dose delivery devices.
WO 2005/018721 describes a pen with a gearing mechanism based on two threads handed in the same direction and a third thread on the piston rod which is not directly a part of the gearing mechanism. A piston rod is connected with a nut. A non-rotational driver is engaging a scale drum via a first thread and the nut via a second thread. The pitch of the first thread is bigger then the pitch of the second pitch and the difference between them is equal to the pitch of the piston rod. When a dose is set, the nut is rotationally locked to the scale drum, and is thereby rotated and elevated a distance corresponding to the elevation of the driver. When the set dose is to be injected, the nut disengages the scale drum to engage the non-rotational driver. As the scale drum is pushed into the device, the rotation of the scale drum will cause the non-rotational driver to retract into the scale drum and the resulting displacement of the driver to be equal to the set dose. The nut is now pushed back to zero position bringing the piston rod along causing insulin to be expelled. It should be noted however that the pitches of the threads are dependant of number of increments per revolution, unit size etc. and the dose force will be relatively high due to the low driving pitch.
In WO 2006/072188 a pen with a similar kind of gearing is described. A scale drum with an outer and an inner thread is engaging the housing via the outer thread and a non-rotational driver via the inner thread. When the scale drum is rotated to set a dose, the scale drum will elevate a certain distance per unit, and the driver will elevate another and smaller distance as it is both elevated directly by the scale drum and rotated back into the pen due to the thread engagement with the scale drum. The driver is engaging a piston rod via a linear one-way ratchet which allows relative movement between the piston rod and the driver when a dose is set, but will carry the piston rod along expelling insulin when the set dose is injected. Like in the previously mentioned concept, the pitches of the threads are dependant of number of increments per revolution, unit size etc. and the dose force will be relatively high due to the low driving pitch. Further more this concept does not allow the user to correct a wrongly set dose.
The disclosure seeks to provide a gearing mechanism comprising a few and simple parts, with a very low injection force and a relative short movement of the injection button during injection.